Thin film transistor array substrate and method for manufacturing the same

ABSTRACT

A thin film transistor array substrate and a method for manufacturing the same is disclosed, in which it is possible to prevent mobile ions contained in a substrate from penetrating into a semiconductor layer by the gettering effect or neutralization in case soda lime glass is used for the substrate. The method includes forming a buffer layer on a substrate; doping impurity ions in the buffer layer; and forming a pixel electrode and a thin film transistor including a semiconductor layer on the buffer layer.

[0001] This application claims the benefit of the Korean ApplicationNos. P2002-85735, filed on Dec. 28, 2002 and P2003-24982, filed on Apr.21, 2003, which are hereby incorporated by reference for all purposes asif fully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a liquid crystal display (LCD)device, and more particularly, to a thin film transistor array substrateand a method for manufacturing the same, in which it is possible toprevent mobile ions contained in a substrate from penetrating into asemiconductor layer by the gettering effect or neutralization in casethe substrate is formed of Low End Glass.

[0004] 2. Discussion of the Related Art

[0005] With the development of an information society, demands forvarious display devices increase. Accordingly, many efforts have beenmade to research and develop various flat display devices such as liquidcrystal displays (LCD), plasma display panels (PDP), electroluminescentdisplays (ELD), and vacuum fluorescent displays (VFD). Some types of theflat display devices are already in use in displays of various types ofequipment.

[0006] Among the various flat display devices, the liquid crystaldisplay (LCD) device has been most widely used due to the advantageouscharacteristics of thinness, lightness in weight, and low powerconsumption. In this way, the LCD device substitutes for the Cathode RayTube (CRT). In addition to the mobile type LCD devices such as displaysfor a notebook computers, LCD devices have been developed for computermonitors and televisions to receive and display broadcasting signals.

[0007] The general LCD device includes an LCD panel for displaying animage and a driver for applying a driving signal to the LCD panel. TheLCD panel includes first and second glass substrates bonded togetherwith a gap therebetween, and a liquid crystal layer injected between thefirst and second glass substrates.

[0008] The first glass substrate (TFT array substrate) includes aplurality of gate and data lines, a plurality of pixel electrodes and aplurality of thin film transistors. The plurality of gate lines areformed on the first glass substrate at fixed intervals in one direction,and the plurality of data lines are formed at fixed intervalssubstantially perpendicular to the plurality of gate lines. Then, aplurality of pixel electrodes in a matrix arrangement are formed inpixel regions defined by the plurality of gate and data lines crossingeach other. The plurality of thin film transistors are switchedaccording to signals on the gate lines for transmitting signals on thedata lines to the respective pixel electrodes. The second glasssubstrate (color filter substrate) includes a black matrix layerpreventing light from leaking into regions except the pixel regions ofthe first substrate, the R/G/B (red/green/blue) color filter layerdisplaying colors, and a common electrode displaying a picture image.

[0009] Next, a gap is maintained between the first and second glasssubstrates by spacers, and the first and second substrates are bonded toeach other by a seal pattern having a liquid crystal injection inlet. Atthis time, the liquid crystal layer is formed using a liquid crystalinjection method, in which the liquid crystal injection inlet is dippedinto a container having liquid crystal while maintaining a vacuum statein the gap between the first and second glass substrates. That is, theliquid crystal is injected between the first and second substrates by anosmotic action. Then, the liquid crystal injection inlet is sealed witha sealant.

[0010] Meanwhile, the substrate for the LCD device is formed of atransparent glass substrate. The transparent glass substrate should benon-alkaline, heat-resistant and chemical-resistant. More particularly,the alkaline elements of the transparent glass substrate cause the thinfilm transistor to deteriorate. Therefore, it is necessary to completelyremove the alkaline elements such as the sodium or potassium groupmaterials in the transparent glass substrate. Also, the substrate isrepetitively heated and cooled during a plurality of semiconductorprocess steps on the substrate. In this respect, it is important toobtain the heat-resistance characteristics in the substrate to stabilizethe size of the substrate in case the glass substrate is heated orcooled. Also, a process for etching a metal or an oxide is required toform a driving device on the substrate for a display operation. Thesubstrate has to obtain stabilization for an etchant of the metal oroxide.

[0011] In manufacturing devices using glass substrate, various kinds ofglass substrates are used according to a manufacturing method asfollows.

[0012] For example, a simple matrix type LCD device is manufactured at alow temperature of 300° C. or less, whereby lower and upper glasssubstrates of the said LCD device are formed of Soda Lime Glass of theinexpensive Low End Glass group. However, in case of an active matrixtype LCD panel having a polysilicon type thin film transistor, a hightemperature process of 300° C. or more is performed to lower and upperglass substrates. Thus, the lower and upper glass substrates are formedof Alumino-Silicate Glass having great resistance to heat damage ortemperature changes and etching-resistance characteristics. TheAlumino-Silicate Glass is three times as expensive as the Soda LimeGlass. That is, as the size of the LCD device increases, the cost ofmanufacturing material such as substrate is a burden.

[0013] The following table shows the remaining amount of sodium ionsaccording to the kind of the glass substrate. TABLE 1 Kind of GlassSubstrate Structure Alumino-Silicate Glass Soda Lime Glass n+/SiNx/GLS(250° C.) a-Si:H 1.3 × 10⁻³ 1.9 × 10⁻² SiNx 2.1 × 10⁻³ 2.6 × 10⁻²n+/SiNx/BCB/ GLS(250° C.) a-Si:H 5.5 × 10⁻⁴ 1.8 × 10⁻³ SiNx 1.4 × 10⁻³3.0 × 10⁻³ BCB(Na/C) 2.2 × 10⁻³ 6.7 × 10⁻² ITO/GLS ITO(Na/In) 8.5 × 10⁻³5.3 × 10⁻² GLS Glass 3.2 × 10⁻¹ 3.6 n+/SiNx/GLS (320° C.) a-Si:H 7.3 ×10⁻⁴ 1.3 × 10⁻² SiNx — —

[0014] As shown in Table 1, in the case of the Low End Glass such as theSoda Lime Glass, respective glass substrates contain a large amount ofsodium as compared to those of the Alumino-Silicate Glass. Even thoughthe temperature of heat treatment is changed in case of the Soda LimeGlass, the amount of sodium ions is scarcely decreased. In fact, theamount of mobile ions remaining on the semiconductor layer (a-Si:H,before hardening to a polysilicon) is the most serious problem becausethe semiconductor layer obtains TFT device characteristics.

[0015] Accordingly, the substrate may be formed of the Soda Lime Glassin a simple matrix type LCD device since it is possible to containmobile ions in a buffer layer of silicon nitride layer SiNx formed onthe glass substrate. However, in case of the active matrix type LCDpanel having an amorphous silicon type thin film transistor requiring ahigh temperature process, it is impossible to use the buffer layer as abarrier layer because the great amount of mobile ions, so the Soda LimeGlass is not used as the substrate in the active type LCD panel.

[0016] Hereinafter, a method for manufacturing a thin film transistorarray substrate of an LCD device according to the related art will bedescribed with reference to the accompanying drawings.

[0017]FIG. 1 is a cross-sectional view illustrating a semiconductorlayer on Low End Glass in an LCD device according to the related art.FIG. 2 is a cross-sectional view illustrating mobile ions of the Low EndGlass penetrating to a semiconductor layer.

[0018] As shown in FIG. 1, in the LCD device according to the relatedart, a buffer layer 12 is formed on a glass substrate 11. The bufferlayer 12 serves as a barrier layer. Then, a semiconductor layer 13 isformed in an island-shape on the buffer layer 12. The buffer layer 12prevents mobile ions of the glass substrate 11 from penetrating into thesemiconductor layer 13.

[0019] Although not shown, a gate insulating layer is deposited on thebuffer layer 12 including the semiconductor layer 13, and a gateelectrode is formed on a predetermined region thereof. Subsequently, aninsulating interlayer is formed on an entire surface of the glasssubstrate 11, and source/drain electrodes are formed and connected to aimpurity region of the semiconductor layer 13, thereby forming a thinfilm transistor. After that, a pixel electrode is formed and connectedto the drain electrode, whereby the thin film transistor array substrateis completed.

[0020] However, as shown in FIG. 2, in case sodium ions Na+ flow intothe glass substrate 11, the buffer layer 12 does not serve as thebarrier layer preventing mobile ions from penetrating into thesemiconductor layer 13. Especially with Low End Glass containing a largeamount of mobile ions, the buffer layer 12 does not serve as the barrierlayer for preventing penetration of mobile ions to the semiconductorlayer 13. Thus, the gate threshold voltage is increased due to themobile ions penetrating into the semiconductor layer 13.

[0021] The method for manufacturing the thin film transistor arraysubstrate according to the related art has the following disadvantages.Recently, the thin film transistors are manufactured at a lowtemperature of 200° C. or less, so that it relaxes the requirements onheat damage or temperature changes. Accordingly, if the substratesatisfies requirements except the heat-resistance requirement, the SodaLime Glass may be used as the substrate for the LCD device. It ispreferable to use the Soda Lime Glass as the substrate because the SodaLime Glass is inexpensive. Soda Lime Glass costs one-third as much asAlumino-Silicate Glass. So if it is possible to prevent thesemiconductor layer from being contaminated by the mobile ions andimpurities of the glass when using the substrate of the Soda Lime Glass,the manufacturing cost will be greatly reduced for the LCD device.However, the buffer layer, formed between the semiconductor layer andthe substrate, does not serve as a barrier layer, whereby the mobileions of the substrate may penetrate into the semiconductor layer. Thus,the gate threshold voltage is increased when forming the thin filmtransistor.

SUMMARY OF THE INVENTION

[0022] Accordingly, the present invention is directed to a thin filmtransistor array substrate and a method for manufacturing the same thatsubstantially obviates one or more problems due to limitations anddisadvantages of the related art.

[0023] An advantage of the present invention is to provide a thin filmtransistor array substrate and a method for manufacturing the same, inwhich it is possible to prevent mobile ions contained in a substratefrom penetrating into a semiconductor layer by the gettering effect orneutralization when Low End Glass is used for the substrate.

[0024] Additional advantages and features of the invention will be setforth in part in the description which follows and in part will becomeapparent to those having ordinary skill in the art upon examination ofthe following or may be learned from practice of the invention. Theadvantages of the invention may be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

[0025] To achieve these advantages and in accordance with the purpose ofthe invention, as embodied and broadly described herein, a method formanufacturing a thin film transistor array substrate includes forming abuffer layer on a substrate; doping impurity ions in the buffer layer;and forming a pixel electrode and a thin film transistor including asemiconductor layer on the buffer layer.

[0026] For example, when the substrate is Soda Lime Glass, the bufferlayer is formed of at least one of a silicon oxide layer, a siliconnitride layer, a silicon oxide nitride layer and an organic insulatinglayer. The impurity ions may be phosphorous ions, and a phospho-silicateglass layer is formed on a surface of the buffer layer for getteringmobile ions penetrating the buffer layer from the outside by doping thephosphorus ions. Also, the impurity ions may be chlorine ions, and areactive layer is formed in an interface between the buffer layer andthe soda lime glass to neutralize mobile ions penetrating the bufferlayer from the outside by doping the chlorine ions.

[0027] In another aspect of the present invention, a thin filmtransistor array substrate may include a glass substrate; a first bufferlayer on the glass substrate; a barrier layer on the first buffer layer;a second buffer layer on the barrier layer; and a thin film transistorarray on the second buffer layer, the thin film transistor array havingat least a semiconductor layer, a thin film transistor and a pixelelectrode. The barrier layer may be formed of a phospho-silicate glasslayer, and the first and second buffer layers may be one of a siliconnitride layer, a silicon oxide layer, a silicon oxide nitride layer, andan organic insulating layer.

[0028] It is to be understood that both the foregoing generaldescription and the following detailed description of the presentinvention are exemplary and explanatory and are intended to providefurther explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this application, illustrate embodiment(s) of theinvention and together with the description serve to explain theprinciple of the invention. In the drawings:

[0030]FIG. 1 is a cross-sectional view illustrating a semiconductorlayer on Low End Glass in an LCD device according to the related art;

[0031]FIG. 2 is a cross-sectional view illustrating mobile ions of theLow End Glass penetrating into a semiconductor layer;

[0032]FIG. 3A is a cross-sectional view illustrating a method formanufacturing a thin film transistor array substrate according to thefirst embodiment of the present invention;

[0033]FIG. 3B is a cross-sectional view illustrating a thin filmtransistor array substrate according to the first embodiment of thepresent invention;

[0034]FIG. 4A is a cross-sectional view illustrating a method formanufacturing a thin film transistor array substrate according to thesecond embodiment of the present invention;

[0035]FIG. 4B is a cross-sectional view illustrating a thin filmtransistor array substrate according to the second embodiment of thepresent invention;

[0036]FIG. 5 is a cross-sectional view schematically illustrating amethod for manufacturing a thin film transistor array substrateaccording to the third embodiment of the present invention;

[0037]FIG. 6 is a cross-sectional view schematically illustrating amethod for manufacturing a thin film transistor array substrateaccording to the fourth embodiment of the present invention;

[0038]FIG. 7 is a cross-sectional view schematically illustrating amethod for manufacturing a thin film transistor array substrateaccording to the fifth embodiment of the present invention;

[0039]FIG. 8 is a cross-sectional view illustrating a top gate type thinfilm transistor on Low End Glass to which a mobile ion treatment isperformed according to one embodiment among the first to fifthembodiments of the present invention; and

[0040]FIG. 9 is a cross-sectional view illustrating a bottom gate typethin film transistor on Low End Glass to which mobile ions treatment isperformed according to one embodiment among the first to fifthembodiments of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0041] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0042] A thin film transistor array substrate and a method formanufacturing the same will be described with reference to theaccompanying drawings.

[0043]FIG. 3A is a cross-sectional view illustrating a method formanufacturing a thin film transistor array substrate according to thefirst embodiment of the present invention. The method for manufacturingthe thin film transistor array substrate according to the firstembodiment of the present invention may include the process steps offorming a first buffer layer 22 on a substrate 21 formed of Low EndGlass, doping phosphorus ions 41 on the first buffer layer 22, andforming a second buffer layer 23 on the first buffer layer 22. Then, apixel electrode (not shown) and a thin film transistor (not shown)including a semiconductor layer 24 may be formed on the second bufferlayer 23, thereby completing the thin film transistor array substrate.The doping process of the phosphorus ions 41 also may be performed afterforming the second buffer layer 23.

[0044] The Low End Glass may be formed of Soda Lime Glass. Also, aphospho-silicate glass layer may be formed at an interface between thefirst and second buffer layers 22 and 23 for gettering mobile ions bythe doped phosphorus ions. Accordingly, the phospho-silicate glass layerserves as a barrier layer to prevent the mobile ions such as Na+ ions ofthe glass substrate from flowing into the semiconductor layer by thegettering effect. The mobile ions such as Na+ ions are dispersed in thesubstrate 21 and the first buffer layer 22.

[0045] The phospho-silicate glass layer may be formed on the surface ofthe first buffer layer 22 formed by the doping process of thephosphorous ions 41, which serves as the barrier layer to prevent themobile ions from penetrating the semiconductor layer 24 during heatprocessing or the processing for forming the thin film transistorincluding the semiconductor layer 24. Also, the second buffer layer 23is additionally may be formed on the first buffer layer in that thephospho-silicate glass layer is formed on the surface of the firstbuffer layer 22. When the semiconductor layer 24 is directly formed onthe first buffer layer 22, the mobile ions, which are not reactive withthe phosphorus ions, may easily flow into the semiconductor layer 24. Atthis time, the first and second buffer layers 22 and 23 may be formed ofa silicon nitride layer SiNx, a silicon oxide layer, a silicon oxidenitride layer, or an organic insulating layer of BCB group.

[0046]FIG. 3B is a cross-sectional view illustrating the thin filmtransistor array substrate according to the first embodiment of thepresent invention, which is manufactured by the aforementioned method.The thin film transistor array substrate according to the firstembodiment of the present invention may include the Low End Glasssubstrate 21, the first buffer layer 22 on the Low End Glass substrate21, the phospho-silicate glass substrate 100 as the barrier layer on thefirst buffer layer, the second buffer layer 23 on the phospho-silicateglass layer 100, the semiconductor layer 24 on the second buffer layer23, and a thin film transistor array (not shown) including the thin filmtransistor and the pixel electrode.

[0047]FIG. 4A is a cross-sectional view illustrating a method formanufacturing a thin film transistor array substrate according to thesecond embodiment of the present invention. The method for manufacturingthe thin film transistor array substrate according to the secondembodiment of the present invention may include the process steps offorming a buffer layer 31 on a substrate 21 formed of Low End Glass,doping chlorine ions 51 into the buffer layer 31, and forming a pixelelectrode (not shown) and a thin film transistor (not shown) containinga semiconductor layer 24 on the buffer layer 31. The Low End Glass maybe formed of Soda Lime Glass, and a reactive layer may be formed in aninterface between the substrate 21 and the buffer layer 31 by the dopedchlorine ions, whereby mobile ions contained in the substrate 21 areneutralized in the reactive layer. Accordingly, the reactive layer maybe formed on the interface of the substrate 21 by doping chlorine ions,whereby the reactive layer serves as a barrier layer to prevent themobile ions from penetrating into the semiconductor layer 24. As aresult, it is possible to prevent the device characteristics fromdeteriorating. The buffer layer 31 may be formed of a silicon nitridelayer SiNx, a silicon oxide layer, a silicon oxide nitride layer or anorganic insulating layer of BCB group.

[0048] The method for manufacturing the thin film transistor arraysubstrate using the chlorine ion injection method according to thesecond embodiment of the present invention is different from thataccording to the first embodiment of the present invention in that thereactive layer is formed on the surface of the substrate 21. As aresult, it is not necessary to additionally form the buffer layer toprevent the mobile ions from penetrating into the semiconductor layer.

[0049]FIG. 4B is a cross-sectional view illustrating the thin filmtransistor array substrate according to the second embodiment of thepresent invention, which is manufactured by the aforementioned method ofFIG. 4A. As shown in FIG. 4B, the thin film transistor array substrateaccording to the second embodiment of the present invention may includethe Low End Glass substrate 21, the buffer layer 31 on the Low End Glasssubstrate 21, the ion reactive layer 200 at the interface between theLow End Glass substrate 21 and the buffer layer 31, the semiconductorlayer 24 on the buffer layer 31, and a thin film transistor arrayincluding the thin film transistor and pixel electrode (not shown).

[0050] As mentioned above, the ion reactive layer 200 may be formed by areaction of chlorine ions and the buffer layer 31, and the buffer layer31 may be formed of a silicon oxide layer, a silicon nitride layer, asilicon oxide nitride layer or an organic insulating layer.

[0051]FIG. 5 is a cross-sectional view schematically illustrating amethod for manufacturing a thin film transistor array substrateaccording to the third embodiment of the present invention. The methodfor manufacturing the thin film transistor array substrate may includethe process steps of forming a first buffer layer 520 on a substrate 510formed of Low End Glass, forming a second buffer layer 530 on the firstbuffer layer 520, doping an impurity such as phosphorus or chlorine ionson the second buffer layer 530, and forming a pixel electrode (notshown) and a thin film transistor (not shown) having a semiconductorlayer 540 on the second buffer layer 530.

[0052] As mentioned above, in the case of doping the phosphorus orchlorine ions after forming the first and second buffer layers 520 and530, the phosphorous or chlorine ions may distribute throughout thefirst and second buffer layers 520 and 530 with a Gaussian profile.Accordingly, if the impurity doping process is performed such that thepeak of Gaussian profile is positioned at the interface between thefirst and second buffer layers 520 and 530, an impurity layer (notshown) of the phosphorous or chlorine ions may be formed in theinterface between the first and second buffer layers 520 and 530. Theimpurity layer prevents mobile ions such as Na or K from moving.

[0053] The first and second buffer layers 520 and 530 may be formed ofthe same material as those of the substrate and buffer layer in thefirst and second embodiments of the present invention. In the method formanufacturing the thin film transistor array substrate according to thethird embodiment of the present invention, the first and second bufferlayers 520 and 530 are formed of a silicon oxide layer. In case ofdoping the phosphorous ions, the impurity layer is formed ofphospho-silicate glass between the first and second buffer layers 520and 530 since an amorphous combination of P—Si—O is the same as a glassstructure.

[0054]FIG. 6 is a cross-sectional view schematically illustrating amethod for manufacturing a thin film transistor array substrateaccording to the fourth embodiment of the present invention. The methodfor manufacturing the thin film transistor array substrate includes theprocess steps of forming a first buffer layer 620 on a substrate 610formed of Low End Glass, forming an reactive layer 630 on an uppersurface of the first buffer layer 620 for preventing mobile ions such asNa and K from moving by performing a plasma treatment using a plasma gassuch as phosphorous or chlorine on the upper surface of the first bufferlayer 620, forming a second buffer layer 640 on the reactive layer 630,and forming a pixel electrode (not shown) and a thin film transistor(not shown) having a semiconductor layer 650 on the second buffer layer640.

[0055] When performing the plasma treatment in the upper surface of thefirst buffer layer 620, the plasma gas including the phosphorous orchlorine penetrates into the first buffer layer 620 at 50 Å or less. Inthe method of manufacturing the thin film transistor array substrateaccording to the fourth embodiment of the present invention, the firstand second buffer layers 620 and 640 may be formed of the same materialas those of the substrate and buffer layer in the first, second andthird embodiments of the present invention.

[0056]FIG. 7 is a cross-sectional view schematically illustrating amethod for manufacturing a thin film transistor array substrateaccording to the fifth embodiment of the present invention. The methodfor manufacturing the thin film transistor array substrate may includethe process steps of forming a buffer layer 720 on a substrate 710formed of Low End Glass, doping an impurity such as phosphorous orchlorine ions on the buffer layer 720, and forming a pixel electrode(not shown) and a thin film transistor (not shown) having asemiconductor layer 730 on the buffer layer 720.

[0057] As mentioned above, in case of doping the phosphorous or chlorineions after forming the buffer layer 720 on the substrate 710, thephosphorous or chlorine ions may distribute throughout the substrate 710and the buffer layer 720 in a Gaussian profile. Accordingly, if theimpurity doping process is performed such that the peak of Gaussianprofile is positioned in an interface between the substrate 710 and thebuffer layer 720, an impurity layer (not shown) of the phosphorous orchlorine ions may be formed at the interface between the substrate 710and the buffer layer 720. The impurity layer prevents mobile ions suchas Na and K from moving. The substrate 710 and the buffer layer 720 maybe formed of the same material as those of the substrate and the bufferlayer in the first, second, third and fourth embodiments of the presentinvention.

[0058] Hereinafter, a method for manufacturing a thin film transistorarray substrate including the thin film transistor with theaforementioned Low End Glass substrate, according to the presentinvention will be described as follows.

[0059]FIG. 8 is a cross-sectional view illustrating a top gate type thinfilm transistor on Low End Glass to which mobile ions treatment isperformed according to one embodiment among the first to fifthembodiments of the present invention. A buffer layer 22 is deposited ona substrate 21. After doping phosphorous or chlorine ions 41 or 51 inthe buffer layer 22, an amorphous silicon a-Si:H is deposited on thebuffer layer 22. When doping the phosphorous ions, a second buffer layer(not shown) may be additionally formed on the buffer layer 22. Afterthat, the deposited amorphous silicon a-Si:H may be dehydrogenated andthen crystallized into a polysilicon by laser. Then, the polysilicon ispatterned, thereby forming a semiconductor layer 24 as an active layerof each thin film transistor (TFT). Subsequently, a gate insulatinglayer 25 is deposited on the entire surface of the semiconductor layer24, and a metal layer is deposited on the gate insulating layer 25.Then, the metal layer is selectively removed, whereby a gate electrode26 of the TFT is formed above a predetermined portion of thesemiconductor layer 24.

[0060] Next, a first photoresist layer is deposited on the entiresurface of the substrate, and a mask (not shown) is formed for highlydoped n-type (n+) impurity ions injection by exposure and developingprocess. Then, the highly doped n-type impurity ions are injectedthrough the mask, thereby forming highly doped n-type impurity regions24 a on the semiconductor layer 24 corresponding to n-type TFT regionand lightly doped drain (LDD) n-type TFT region. That is, a first mask(not shown) is formed on p-type TFT region for covering an entiresurface of the p-type TFT region, and a second mask (not shown) isformed on the LDD n-type TFT region, the second mask (not shown) iswider than the gate electrode 26. The second mask is formed on the gateinsulating layer 25 to have a width suitable for covering the gateelectrode 26 and LDD region formed in the following process. Also, thegate electrode 26 serves as the mask in the n-type TFT region, and thehighly doped n-type impurity regions 24 a are formed in thesemiconductor layer 24 at both sides of the gate electrode 26.Generally, the n-type ion material is PH3.

[0061] Subsequently, the first and second masks are removed. A secondphotoresist layer is deposited, and then an exposure and developingprocess is performed thereon, so that a third mask (not shown) is formedfor highly doped p-type (p+) impurity ion injection. Then, the highlydoped p-type impurity ions are injected through the third mask, wherebyhighly doped p-type impurity regions 24 b are formed on thesemiconductor layer 24 corresponding to p-type TFT region. The thirdmask (not shown) covers the entire surface of the n-type TFT region andLDD n-type TFT region, and then the highly doped p-type impurity ionsinjection process is performed in the p-type TFT region by using thegate electrode 26 as the mask. Generally, the p-type ion material isB2H6.

[0062] Next, the third mask is removed, and then lightly doped n-type(n−) impurity ions are injected by using the gate electrode 26 of theTFT region as the mask. At this time, lightly doped n-type impurityregions 24 c are formed in the semiconductor layer 24 corresponding toboth sides of the gate electrode 26 of the LDD n-type TFT region. Inthis case, the lightly doped n-type impurity ions injection process doesnot have a great effect on the impurity intensity of the highly dopedimpurity regions such as the n-type TFT region and p-type TFT regionsince the n-type or p-type TFT region is formed through the highly dopedimpurity ions injection process, the preceding process. Also, it ispossible to change the order of the lightly doped ions injection processand the highly doped ions injection process. The lightly doped ionsinjection process uses the gate electrode as the mask, and the highlydoped ions injection process uses a mask for covering the LDD region.

[0063] After doping, an insulating interlayer 29 is formed on the entiresurface of the substrate, and then the insulating interlayer 29 and thegate insulating layer 25 are selectively removed to form contact regionsbeing contacted with the highly doped impurity regions 24 a and 24 b ofthe semiconductor layer 24. Subsequently, a metal layer is deposited onthe entire surface of the insulating interlayer 29 including the contactregions, and then source and drain electrodes 30 are formed bypatterning the metal layer. Although not shown, a passivation layer (notshown) may be formed on the entire surface of the substrate 21 includingthe source and drain electrodes 30, and a pixel electrode (not shown)may formed on a predetermined portion of the passivation layer for beingconnected to the drain electrode 30.

[0064] In addition to the top gate type thin film transistor for formingthe semiconductor layer on the buffer layer, a bottom gate type thinfilm transistor is proposed to prevent alkaline ions from the glasssubstrate from penetrating into the device. In the bottom type thin filmtransistor for forming a semiconductor layer above a gate electrode,chlorine or phosphorous ions are doped on a buffer layer for preventingthe alkaline ions from penetrating into the inside of the device.

[0065] Hereinafter, a method for forming the bottom type thin filmtransistor will be described. In the method for forming the bottom typethin film transistor, mobile ion treatment is performed on the bufferlayer.

[0066]FIG. 9 is a cross-sectional view illustrating the bottom gate typethin film transistor on Low End Glass to which a mobile ion treatment isperformed according to one embodiment among the first to fifthembodiments of the present invention. A buffer layer 62 is deposited ona substrate 61. Then, phosphorus 41 or chlorine 51 ions are doped on thebuffer layer 62 for gettering alkaline ions such as sodium ions in thebuffer layer or interface between the substrate 61 and the buffer layer62. Then, a metal layer is deposited on the entire surface of the bufferlayer 62, and then selectively removed to form gate electrodes 63 ofrespective TFTs. A gate insulating layer 64 is deposited on the entiresurface of the buffer layer 62 including the gate electrodes 63.Subsequently, a semiconductor layer 65 is formed on the gate insulatinglayer 64. The semiconductor layer 65 may be formed by depositingamorphous silicon a-Si:H, dehydrogenating the deposited amorphoussilicon (a-Si:H), and crystallizating the dehydrogenated amorphoussilicon. In this case, the crystallization process may be performedbefore or after patterning the semiconductor layer 65.

[0067] As mentioned above, impurity ions are injected to thesemiconductor layer 65 corresponding to the respective TFT regions,thereby forming impurity regions 65 a, 65 b and 65 c. After that, ametal layer is deposited on the entire surface of the semiconductorlayer 65, and then selectively removed to form source/drain electrodes66. Like FIG. 8, although not shown, a passivation layer (not shown) maybe formed on the entire surface of the substrate 61 including thesource/drain electrodes 66, and then a pixel electrode (not shown) maybe formed on a predetermined portion of the passivation layer for beingconnected to the drain electrode 66.

[0068] As mentioned above, the thin film transistor array substrate andthe method for manufacturing the same according to the present inventionhas the following advantages. First, the substrate is formed of cheapLow End Glass, thereby decreasing manufacturing cost. Also, thephosphorous or chlorine ions are doped on the buffer layer, so that itis possible to prevent the mobile ions from penetrating into thesemiconductor layer by the gettering effect or neutralization of thereactive layer, thereby forming the thin film transistor and obtainingstable device characteristics.

[0069] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for manufacturing a thin film transistorarray substrate comprising: forming a buffer layer on the substrate; andplacing impurity ions in the buffer layer; and forming a pixel electrodeand a thin film transistor including a semiconductor layer on the bufferlayer.
 2. The method of claim 1, wherein the substrate is a soda limeglass substrate.
 3. The method of claim 1, wherein the buffer layer isformed of at least one of a silicon oxide layer, a silicon nitridelayer, a silicon oxide nitride layer, and an organic insulating layer.4. The method of claim 1, wherein the impurity ions are one ofphosphorous ions and chlorine ions.
 5. The method of claim 1, wherein aphospho-silicate glass layer is formed on a surface of the buffer layerfor gettering mobile ions penetrating the buffer layer by the placingphosphorus ions in the buffer layer.
 6. The method of claim 5, furthercomprising another buffer layer on the buffer layer including thephospho-silicate glass layer.
 7. The method of claim 1, wherein theimpurity ions are placed forming a Gaussian profile of impurity ions. 8.The method of claim 7, wherein the peak of the Gaussian profile ofimpurity ions is positioned at an interface between the glass substrateand the buffer layer.
 9. The method of claim 1, wherein the impurityions are chlorine ions and a reactive layer is formed at an interfacebetween the buffer layer and the substrate to neutralize mobile ionspenetrating the buffer layer.
 10. The method of claim 1, wherein placingincludes one of implantation, plasma treatment, doping, ion showering,diffusion, chemical vapor deposition, sputtering, and dry etching.
 11. Amethod for manufacturing a thin film transistor array substratecomprising: forming a first buffer layer on a glass substrate; forming asecond buffer layer on the first buffer layer; and forming a reactivelayer between a surface of the glass substrate and an outer surface ofthe second buffer layer; forming a pixel electrode and a thin filmtransistor including a semiconductor layer on the second buffer layer.12. The method of claim 11, wherein placing includes one ofimplantation, plasma treatment, doping, ion showering, diffusion,chemical vapor deposition, sputtering, and dry etching.
 13. The methodof claim 11, wherein the reactive layer is formed by placing impurityions in the first buffer layer.
 14. The method of claim 11, wherein thereactive layer is formed by placing impurity ions in the second bufferlayer.
 15. The method of claim 11, the substrate is a soda lime glasssubstrate.
 16. The method of claim 11, wherein the first and secondbuffer layers are formed of any one of a silicon oxide layer, a siliconnitride layer, a silicon oxide nitride layer, and an organic insulatinglayer.
 17. The method of claim 11, wherein the impurity ions are placedforming a Gaussian profile of impurity ions.
 18. The method of claim 17,wherein the peak of the Gaussian profile of impurity ions is positionedat an interface between the first and second buffer layers.
 19. Themethod of claim 17, wherein the peak of the Gaussian profile of impurityions is positioned at an interface between the first buffer layer andthe glass substrate.
 20. The method of claim 11, wherein the reactivelayer is a phospho-silicate glass layer formed by doping phosphorousions for gettering mobile ions penetrating from the glass substrate atan interface between the first buffer layer and second buffer layer. 21.A thin film transistor array substrate comprising: a glass substrate; abuffer layer on the glass substrate; a reactive layer between a surfaceof the glass substrate and an outer surface of the buffer layer; and athin film transistor array on the second buffer layer, the thin filmtransistor array having at least a semiconductor layer, a thin filmtransistor and a pixel electrode.
 22. The thin film transistor arraysubstrate of claim 22, wherein the reactive layer is formed by areaction of one of chlorine and phosphorous and the buffer layer. 23.The thin film transistor array substrate of claim 21, wherein the bufferlayer is one of a silicon nitride layer, a silicon oxide layer, asilicon oxide nitride layer, and an organic insulating layer.
 24. Themethod of claim 21, wherein the substrate is a soda lime glasssubstrate.
 25. The method of claim 21, wherein the reactive layer is aphospho-silicate glass layer is formed on a surface of the buffer layerfor gettering mobile ions penetrating the buffer layer by the placingphosphorus ions in the buffer layer.
 26. The method of claim 1, whereinthe reactive layer is formed by placing impurity ions in the bufferlayer forming a Gaussian profile of impurity ions.
 27. The method ofclaim 26, wherein the peak of the Gaussian profile of impurity ions ispositioned at an interface between the glass substrate and the bufferlayer.
 28. A thin film transistor array substrate comprising: a glasssubstrate; a first buffer layer on the glass substrate; a second bufferlayer on the first buffer layer; a reactive layer between a surface ofthe glass substrate and an outer surface of the second buffer layer; anda thin film transistor array on the second buffer layer, the thin filmtransistor array having at least a semiconductor layer, a thin filmtransistor and a pixel electrode.
 29. The thin film transistor arraysubstrate of claim 28, wherein the barrier layer is formed of aphospho-silicate glass layer.
 30. The thin film transistor arraysubstrate of claim 28, wherein the first and second buffer layers areone of a silicon nitride layer, a silicon oxide layer, a silicon oxidenitride layer, and an organic insulating layer.
 31. The thin filmtransistor array substrate of claim 28, wherein the reactive layer isformed by one of the following implantation, plasma treatment, doping,ion showering, diffusion, chemical vapor deposition, sputtering, and dryetching.
 32. The thin film transistor array substrate of claim 28,wherein the reactive layer is formed by placing impurity ions in thefirst buffer layer.
 33. The thin film transistor array substrate ofclaim 28, wherein the reactive layer is formed by placing impurity ionsin the second buffer layer.
 34. The thin film transistor array substrateof claim 28, the substrate is a soda lime glass substrate.
 35. The thinfilm transistor array substrate of claim 28, wherein the reactive layeris formed by placing impurity ions in a Gaussian profile of impurityions.
 35. The thin film transistor array substrate of claim 36, whereinthe peak of the Gaussian profile of impurity ions is positioned at aninterface between the first and second buffer layers.
 37. The thin filmtransistor array substrate of claim 28, wherein the peak of the Gaussianprofile of impurity ions is positioned at an interface between the firstbuffer layer and the glass substrate.
 38. The thin film transistor arraysubstrate of claim 28, wherein the reactive layer is a phospho-silicateglass layer formed by doping phosphorous ions for gettering mobile ionspenetrating from the glass substrate at an interface between the firstbuffer layer and second buffer layer.
 39. A method for immobilizingalkali ions from a glass substrate comprising: forming a buffer layer onthe glass substrate; and placing impurity ions in the buffer layer. 40.The method of claim 39, wherein the substrate is a soda lime glasssubstrate.
 41. The method of claim 39, wherein the buffer layer isformed of at least one of a silicon oxide layer, a silicon nitridelayer, a silicon oxide nitride layer, and an organic insulating layer.42. The method of claim 39, wherein the impurity ions are one ofphosphorous and chlorine ions.
 43. The method of claim 39, wherein theimpurity ions are placed forming a Gaussian profile of impurity ions.44. The method of claim 43, wherein the peak of the Gaussian profile ofimpurity ions is positioned at an interface between the glass substrateand the buffer layer.
 45. The method of claim 39, wherein placingincludes one of implantation, plasma treatment, doping, ion showering,diffusion, chemical vapor deposition, sputtering, and dry etching.